2^4 Roy’s Account of 
In other Rates of the atmofphere, it varies with the height of 
the barometer, and inverfely with that of the thermometer 
augmented by the number 350 to the number 400. 
It is therefore obvious, that terreftrial objects as well as 
celeftial, rauft fuffer a refradlion greater or lefs, according as 
they are lefs or more removed from the horizon ; and thatfup- 
pofing celeftial refradlion to be perfedlly aicertained, the mea- 
fure of the lower part of any of its curves, co-inciding with 
a particular objedl on the furface of the earth, fhould give the 
quantity by which the apparent altitude of that objedl would 
exceed its real altitude, or what would be its angle of eleva- 
tion, if no fuch effedl as refradlion did exift in nature. 
The inftrument made ufe of in the triangular operation was 
extremely well calculated, as will be remembered from its 
defcription, for meafuring with much exactnefs fmall angles of 
elevation or depreliion, and confequently was in that rcfpedt 
very fit for the purpcfe, if the multiplicity of other bufinefs 
we had on our hands at the time had permitted refraction to 
become a primary, inftead of being only a fecondary objedl. 
This will readily be conceived by thofe who have any idea of 
the trouble of condudting, efpecially at a late feafon of the 
year, an operation of the nature of that in which we were 
engaged. Along with the lights on the French fide of the 
Channel, we had by day as well as by night our own inland 
obfervations to attend to; the very circulation of orders to the 
men ported at the different Rations from twelve to fifteen or 
twenty miles off, in different diredlions around the horizon, 
when any part of the arrangements failed, fo as to render a 
repetition of lights neceffary, was not a matter of fmall 
detail* 
2 
But 
